mode.The mode option is a safe-guard that prevents occasional cross-origin requests:
"cors"… Fetch: Cross-Origin Requests,
"same-origin" – cross-origin requests are forbidden,
"no-cors
That policy is called “CORS”: Cross-Origin Resource Sharing.
Why is CORS needed?… CORS for safe requests.If a request is cross-origin, the browser always adds the Origin header to it.
Similar cross-origin policy (CORS) is enforced for other types of resources as well.
External scripts that are fetched from another origin (e.g. another site) require CORS… To load external scripts from another origin (domain/protocol/port), CORS headers are needed.
Cross-origin requests.XMLHttpRequest can make cross-origin requests, using the same CORS